1. Field of the Invention
The present invention relates to multi-speed gearset transmissions, and in particular, transmissions useful in agricultural tractors.
2. Description of the Related Art
In use, the engine throttle on an agricultural tractor normally is set at a predetermined level, and ground speed is adjusted by changing gears in the transmission. Agricultural tractors therefore require relatively small speed changes between gears (e.g., less than a 1 kilometer per hour difference) during field or tillage use to maintain near constant ground speed at varying loads, or to maintain maximum power output for maximum productivity.
"Gear spacing" is the ratio difference between gears, which produces the change in vehicle speed when the vehicle operator shifts to a different gear. It is common practice for agricultural tractor transmissions to provide approximately equal ratio spacings between all gears. This usually is dictated by the inherent capability of the transmission design. Since both cost and operator acceptance limit the maximum number of gears in a transmission, it is common to compromise between close ratio steps between gears (providing more speeds) and the total speed difference between the first gear and the last gear. Tractors with good field-working gears therefore often have a top speed which is slower than the operator's desired transport speed, or a first gear which is too fast for some operations.
"Shift quality" is the operator's perception of how smoothly a transmission reacts when making a shift. Shift quality is subjectively measured by the operator as a function of acceleration, cab vibrations, and noise or shift clunk. These variables are heavily influenced by two major factors: First, the number of clutch changes made during the shift; second, the gear ratio difference associated with each clutch shifted.
It is common practice in agricultural transmissions to have gear shifts involving three pairs of clutches with at least one clutch having a very large ratio change, resulting in a transmission which does not shift smoothly. These three-clutch shifts most often occur in the tillage range. Since the tillage range is precisely where an operator is most likely to shift frequently (to obtain very close speeds under varying load conditions), this undesirable condition has a high frequency.
Transmission efficiency is influenced primarily by the number of gear meshes carrying the power through a transmission. A common rule of thumb is that each gear mesh consumes 1.5% of the power being transmitted. Current designs of power shift transmissions in agricultural tractors transmit the power through three to seven gear meshes between the engine and the spiral bevel output pinion. This 4.5 to 10% power loss costs the customer additional fuel and requires a larger cooling system than would be required with a more efficient transmission.